


Serendipity; Zemblanity

by RedEyedRyu



Category: Swapfell - Fandom, Undertale (Video Game)
Genre: Additional Tags to Be Added, Alternate Universe - Swapfell, Characters Added As They Appear - Freeform, Dark fic, Explicit Sexual Content, F/M, Kidnapping, Non-Consensual Drug Use, Non-Consensual Somnophilia, Obsessive Behavior, Possessive Behavior, Power Imbalance, Rape/Non-con Elements, Somnophilia, Soul Sex, Stalking, Unhealthy Relationships, Yandere Sans, dark themes, fastburn, physical violence, soul manipulation
Language: English
Status: In-Progress
Published: 2020-02-11
Updated: 2020-11-24
Packaged: 2021-02-27 20:13:58
Rating: Explicit
Warnings: Graphic Depictions Of Violence, Rape/Non-Con
Chapters: 4
Words: 2,689
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/22661563
Author URL: https://archiveofourown.org/users/RedEyedRyu/pseuds/RedEyedRyu
Summary: He keeps running into you—again, and again,and again. And each time, you wind up doing something that catches him completely off-guard. Humans aren't supposed to be this kind to monsters; humans aren't supposed to be thisnice.But you... There's something about you.Each time he finds himself watching you, you wind up doing something to further endear yourself to him. At the same time, however, it has him thinking: this human is far too reckless. You keep putting yourself in dangerous situations that threaten your safety, and as he begins to fall deeper into his obsession with you, he finds himself thinking he needs to fix that.| A dark Swapfell drabble fic |
Relationships: Sans (Swapfell)/Reader
Comments: 42
Kudos: 365





	1. Initial Contact

**Author's Note:**

> Whelp. Here we are again with yet another (dark) fic. Gomen. m(_ _)m This time we're focusing on a yandere Black.
> 
> I've been talking about this idea on tumblr, as well as initially having posted this first snippet there, and well... with the response I've been getting, I figure I might just go ahead and make this an actual thing. Don't know about any kind of update frequency, and chapters will be fairly short, so bare that in mind. This will also be a _very_ dark fic with dark, heavy themes and subject matter, so **make sure you heed the warnings and tags before proceeding!** This first chapter isn't too bad but don't expect this lightness to continue for long.

The both of you are riding the city bus, rows apart and completely unaware of one another, when it stops to let more passengers on. There’s a monster—a haggard looking leporid with creamy fur that fades to a dark mocha in various spots—with a swaddled baby in her arms among the new passengers. Unfortunately there are no open seats by the time she boards. The monster is looking imploringly at people to let her sit as she shifts her hold on her child, her appearance making it obvious how worn and tired she is, but everyone is either acting as if they don’t see her or making it so they take up more space—one person going so far as to pull their backpack from the floor and onto a space beside them that could have fit her with ample room to spare.

Sans is about ready to get up and offer the monster his seat when you, seated a couple rows in front of him, beats him to it. You stand and call out to the monster: “Ma'am! Please, have my seat.” You take a moment to glare heatedly at your fellow human passengers as you shift yourself. Some look away, the guilt obvious in their stooping posture and dipping heads, but none of them meet your eyes.

The monster tries to protest, caught off guard by the unexpected kindness, but you simply reassure her.

“No, no,” you start, waving a dismissive hand at her, “don’t worry about it! It’s the least I could do!“ And some of the other passengers shuffle or clear their throats uncomfortably but otherwise remain silent.

The monster, seeing that you’re set in your decision and have sincere intentions, takes the seat as you stand beside her. Sans watches with a tight and wary expression as the two of you make polite, friendly conversation. He settles back in his seat, having been caught in the middle of acting. His sockets squint at you, his rubine eyelight analyzing all that he can see of you. He’s not used to seeing that kind of treatment from humans towards monsters. It’s abnormal. _Suspicious._

He makes sure to watch you for the duration of the monster’s ride, going so far as to miss his own stop, but no, you are polite and friendly throughout the whole ride, going so far as to help the monster off the bus. The two of you part ways with a shared smile and wave.

It’s a… _curious_ encounter and you are an equally curious human, that much Sans will allow himself to admit, but it’s not like he will see you again. He has a sneaking suspicion that had not been your intended stop, either, but he has no way of verifying his suspicion without following you. Something that would require far more time and attention than he has to spare for a signal, random human. He brushes the incident aside as a one time thing.

…until he runs into you once more.

**Notes for the Chapter:**

> Stay tuned for more bad times! :v  
> In the mean time, feel free to screech at me over [on tumblr](https://redeyedryu.tumblr.com/)!  
> And before I go, here's a sketch of our yandere boyo~


	2. Second Contact

The second time he sees you, he starts to grow suspicious. Just what are the chances of the two of you running into each other again, and so soon after the bus incident no less? A month or more isn't too surprising, living in the same city and all, but a _week_? Sans finds it hard to believe that there isn't something... about this—about _you_.

Perhaps he shouldn't have been so presumptuous to have so easily written you off.

Sure it was an innocuous encounter, much like your first meeting, and admittedly you don't seem to be aware of the skeleton's presence this time either; however, as he watches you grab something from the shelf far out of the reach for the little mouse monster all but completely wrapped in that gaudy striped scarf, he finds himself squinting at the scene.

Did you plan this? Humans don't make it a habit to frequent this particular grocery store—not since they started opening their doors to monsters. And the fact you're helping a monster child...

_My, my, aren't you a devious little human?_

Sans's near perpetual grin grows sharp as he continues to watch you. You're talking to the mouse—a monster he remembers seeing skulking around Snowdin back when they had all still remained locked beneath the mountain. Just another weak and cowardly waste of dust and magic. And yet... you must be quite the piece of work to be able to feign such an expression: your cheeks plump with how wide you're smiling, the way your eyes light up as you laugh, the way you shift your body. You're _almost_ able to make it look as if you're actually enjoying whatever inane conversation the two of you seem to be holding, but Sans knows better. Humans can't be trusted; surely you _must_ have _some_ kind of ulterior motive. As he watches you wave to the mouse and turn to leave, adjusting your hold on the basket cradled against your side, he decides that if you keep this up, he might just have to keep an eye socket on you after all.

**Notes for the Chapter:**

> Come scream into the void with me over on tumblr: [redeyedryu](https://redeyedryu.tumblr.com/)


	3. Third Contact

**Summary for the Chapter:**

> He's spiraling.

The third time he runs into you lasts no longer than a split second, a serendipitous crossing of paths on the escalator at the mall he frequents most. He's scanning the crowd as he often does because it just won't do for the captain of the royal guard to _ever_ let his guard down—to let himself be caught unaware—when his rubine gaze settles on _you_. You're riding up as he descends, having just left his favorite boutique. You're clearly attempting to act as if you _aren't_ trying to sneak up on one of the most infamous monsters the Underground has ever known, your face trained on the phone in your hand. That soft smile... the way the corners of your eyes crease and the light shake of your shoulders as you try to suppress a laugh... You're not fooling him. A hissed " _TCH_ " slips from Sans as his jaw sets in a scowl, body tensing in preparation for you to make your move.

He _knew_ you were a crafty little wench. To be so intent on challenging him that you would go this far: seeking him out in a place with so many uncontrollable variables.

"m'lord?" Papyrus shuffles from his position on the step behind him but Sans pays his brother no heed.

He waits for the moment when you're most likely to strike, his legs shifting to better brace for whatever you're undoubtedly planning to throw at him. The two of you slowly draw closer... and closer... and closer yet. He watches, his magic thrumming just beneath the surface, just _waiting_ for an excuse to be unleashed.

Closer... closer... and closer still, until you could easily reach right across and make a grab for him, but.... Nothing happens. Sans merely continues his descent and you continue your ascent, face still completely _engrossed_ in your phone, completely unaware to the world—to _~~him~~_ the people—around you. And Sans.... Sans doesn't understand. This... this makes no sense.

_What are you doing? Why aren't you doing anything?!_

When he reaches the bottom he's quick to whirl around, his expression set in a scowl and his magic all but crackling around him. It's just as Papyrus settles at his side, once more calling out to him, that Sans spots you, walking along the glass railing bridging the gap between the left and right sections of the upper floor. Your focus is _still_ on that _stupid cell phone of yours._

~~Why aren't you looking at him? _You should be looking at him._~~

He makes a sound reminiscent of clicking one's tongue and, practically _seething_ , turns and speeds towards the exit, leaving his brother to stare after him.

Papyrus's bony brow furrows, completely perplexed by his brother's odd behavior. He turns and looks towards the second level of the mall, gaze chasing after whatever had so clearly perturbed him. When he fails to see anything that might warrant such a reaction from Sans, he shrugs and shuffles on. If it's important Sans will let him know.

**Notes for the Chapter:**

> Oh _boi_.


	4. Fourth Contact

**Summary for the Chapter:**

> Why won't you just look at him?

Sans is... confused. Or perhaps it would be more accurate to say he's _annoyed_.

He doesn't know what to make of you, the human that keeps injecting herself into his life. At first he thought you were just like any other human: conniving, motivated by greed and malice and hate. That you were trying to stupidly take on one of the strongest, most notable monsters to surface because doing so would undoubtedly bring some kind of prestige and notoriety among whatever group you belonged to, but the longer he watched you, the deeper he dug... the less and less sense you made. And the more he began to actually question _himself_. Which is something he just _does. not. do._ Ever.

And yet... and yet... _no one is that good at playing pretend_ —at covering their tracks.

He's dug and dug and _dug_ but he just can't find any dirt on you. No connections to those vexing groups vying to undo any and all progress that the monsters have made in recent years. Hell, he hadn't even found a single off-hand, negative or racist comment aimed at monsters in _any_ of your social media accounts. And he's gone back _years_ , back to even before monsters had surfaced, and _nothing!_

You don't make any _sense_. He _hates_ it. He thinks... maybe he hates _you_ , too. But then he remembers those smiles and soft expressions, the way your eyes lit up when you spoke with those monsters—the same way they do when you speak with your friends and your parents.

~~He wouldn't mind you looking at _him_ like that.~~

Sans feels conflicted when he realizes he has to come to terms with the fact that maybe he was wrong about you. You aren't some conniving human with dastardly laid plans setting some kind of trap for him. The more he watches you, the more he learns about you... It's a hard pill to swallow but it looks like you really are just a kindhearted idiot unwilling to go against your own morals, to ignore the unfair treatment of those around you—whether they be human or monster. It would be admirable if you weren't so weak. So impulsive and _stupid_.

It's the fourth time the two of you cross paths and Sans isn't... he's _not_ upset that you still don't look his way, that you don't acknowledge his presence, ~~that you aren't looking at _him_~~. After this particular incident, however, Sans _is_ certain that things need to change and they need to change _fast_ , lest you wind up getting yourself hurt—or worse.

Headstrong, overly self-confident and stubborn woman that you are, you're _far_ too unaware of just how easy it would be for someone to hurt you.

Sans just so happens to be seated in a far corner of the diner you frequent most often (a quaint, local mom and pop type of place) and if he's angled in just such a way to watch your every move, to see the way your cheeks pillow when you react to something on your phone (a message from a friend? a family member? ~~he wants to message you, too, to see you smile like that for _him_~~ ) well... _one_ of you has to be vigilant in this relationship.

You're seated at the bar counter that loops around the eatery, sipping at a milkshake and absently munching on the fries that remain from your burger combo meal when it happens. The bell just above the diner's door jingles as a middle-aged man (Caucasian, slightly overweight, clothes threadbare and covered in grease stains, carries himself like a buffoon and appears to favor his left foot) shuffles in. His face is tense and Sans can easily tell this is a man on the hunt for trouble. He's proven right when the human's bloodshot eyes home in on a monster couple seated in a booth only a few feet from the entrance.

A quick glance and Sans can easily tell how several people have already picked up on the hostility in the air, can deduce what this man is after. Yet they all avert their eyes and pretend they don't notice. Except you.

The man immediately starts in on the monsters, who visibly shrink in on themselves in a poor attempt to make themselves smaller.

Sans's sockets narrow ever so slightly as he takes in the way your shoulders pull in, the way your overall posture tightens and how you're gripping your milkshake so hard the tips of your fingers are tinted with a white-knuckled grip. A bomb fit to burst.

The fuse is lit as the man wastes no time in attacking the monsters with vitriolic hatred, spit flying as he rages.

Sans watches, posture tense and teeth clenched, silently willing you to just _sit back down_ and _behave_ , but of course, you don't listen. You shift, making as if to stand, and he watches as you purposefully trip over nothing, the cup in your hand tilting just so that its remnants spill directly onto the crotch of the man. He yelps, jumping back a few inches as the remnants of the icy-cold liquid quickly seeps into the denim fabric, and you expertly follow through with your bit—clumsily righting the spilled glass, frantically gathering up napkins and handing them to the man as you anxiously apologize.

"Shit!" you exclaim. "Oh my god, I'm so sorry!" you babble, expression expertly schooled to that of a flustered, embarrassed mess, the words falling from your lips in panicked, speedy strings. Sans would almost believe your remorse, your clever little act, if he didn't know any better—if he didn't know _you_ any better. "Are you okay? I'm so clumsy, I am so, _so_ sorry, oh my-!"

The man cuts you off, shoving you away with enough force that you stumble back and tumble to the ground as he curses at you. You fall against a nearby stool, arms shooting out behind you as you catch yourself. The stool isn't so lucky as it clatters to the floor in a noisy crash and it's as if that functions as a catalyst. The humans that had previously been passive, content to watch this man terrorize innocent monsters and play the ignorant fool, seem to have a different response when a lone human female is involved.

A man seated at a nearby stool shoots up, shifting to stand between you and the offender. "Hey, man!"

A young couple rushes to you, one crouched along your side, supporting your back as her girlfriend offers you a hand.

Sans's eye-light darkens, the grip on his coffee cup tightening. He's equal parts annoyed at the lout of a man and the other humans in the establishment, unwilling to act until one of their own is threatened. How self-serving, their worry and motivation so obviously conditional—unlike you, who opens her heart to anyone, ~~to those that don't deserve it~~. He takes a heavy inhale and pushes the welling magic in his soul down, consciously smooths his expression to that of neutral indifference. _Don't give yourself away_ , he tells himself, _now is not the time_.

With a long practiced control, he reigns in his rising emotions and continues what he does best: he watches. Observes as the man, now outnumbered and facing more of a fight than he's prepared for, begrudgingly backs down, face as red as a tomato, muttering curses beneath his breath. He books a hasty retreat, tossing open the diner's door with a seething glare directed at you, spouts some cliché line about how, "You best watch your back, girlie." And then he's gone, the palpable tension in the area leaving with him. Sans continues to watch as your fellow humans help you resettle, as you help the waitress clean up the mess, as you check in on the monsters that had been terrorized. 

Yes, Sans affirms, things need to change.

But first... he has something he must take care of.

**Notes for the Chapter:**

> Time to take out the trash.


End file.
